1. Field of the Invention
The present invention generally relates to hologram scales and, more particularly, to a reflection type hologram scale suitably applied to a moving amount detecting apparatus, a measuring machine or a displacement measuring apparatus, for example.
2. Description of the Prior Art
Conventionally, hologram scales are utilized in an optical, precision displacement measuring apparatus. Japanese Laid-Open Patent Publication No. 63-75518, for example, describes one of such hologram scales. This type of hologram scale will be described below.
FIG. 1 of the accompanying drawings shows an example of a conventional reflection type hologram scale in which diffracting slits 32 are provided on one surface of a glass substrate 31 at an equal pitch and an evaporated mirror 33 is provided on the other surface of the glass substrate 31.
In the prior-art reflection type hologram scale shown in FIG. 1, the diffracting slits 32 and the evaporated mirror 33 are both exposed to the air so that the diffracting slits 32 are smudged by fingermarks, dust, liquid or the like and that the evaporated mirror 33 is easily scratched.
When the diffracting slits 32 are smudged by fingermarks, dusts or liquid and so on or when the evaporated mirror 33 is scratched, then a diffracted light is not produced by a light incident on the diffracting slits 32 or a primary diffracted light incident on the glass substrate 31 is not reflected by the evaporated mirror 33 with the result that the displacement cannot be measured. Further, when fingermarks, dust, liquid or the like on the diffracting slits 32 are removed, then the diffracting slits 32 are easily scratched.